Recently, in order to adapt to cordless electrical and electric equipments, non-aqueous batteries, such as lithium batteries (lithium primary batteries) and lithium-ion secondary batteries (lithium-ion rechargeable batteries), have attracted attention as a power source with lightweight from which high electromotive force and high energy are generated. For example, tubular-type lithium secondary batteries and others have been produced in large quantities to be used for cellular phones, laptop computers, and the like, so that the production volume has been increasing year by year. Furthermore, the non-aqueous batteries have also attracted attention as an energy source for next-generation electric motorcars. There also has been a growing desire for higher output power achieved by inhibiting electric resistance at a higher level.
There has been known, as an electrolyte for this type of non-aqueous batteries, a solution comprising an organic solvent (such as ethylene carbonate, propylene carbonate, acetonitrile, γ-butyrolactone, 1,2-dimethoxyethane and tetrahydrofuran) and an electrolyte (such as LiPF6, LiCF3SO3, LiClO and LiBF4) dissolved in the organic solvent. However, these electrolyte solutions have a danger of ignition at a high temperature.
Furthermore, in the case where abnormal current flows in the non-aqueous batteries comprising above-mentioned materials by a short circuit or incorrect connection between positive and negative electrodes, such abnormal current may raise battery temperature to a notable degree. As a result, the high temperature of the battery may not only give a thermal damage to the instrument incorporating the battery but also may cause ignition of the electrolyte solution.
Accordingly, paying attention to a paper excellent in heat-resisting property, Patent Document 1 (JP Laid-open Patent Publication No. 8-306352) discloses a non-aqueous battery characterized in using a paper as a separator which intervenes between electrodes, the paper made by using 10% by weight or more of beaten raw material made of beatable regenerated cellulose fibers.
According to this invention, it is possible to obtain a non-aqueous battery having an increased battery capacity with high performance by using a thin separator not only excellent in heat resistance and ionic permeability but also excellent in insulation.
Moreover, Patent Document 2 (JP Laid-open Patent Publication No. 2006-92829) discloses a separator for lithium-ion secondary batteries comprising a network sheet and fiber aggregates laminated on both sides of the sheet, the fiber aggregates having a fiber diameter of 2 μm or smaller and a fiber length of 20 μm or longer.
According to this document, it is possible to obtain a separator for lithium-ion secondary batteries which is excellent in discharge rate and has overcharge resistance.